Exhaust gas purification system and method for internal combustion engine

ABSTRACT

An exhaust gas purification system for an internal combustion engine ( 1 ) is provided which comprises a Nox trap catalyst ( 13 ) and an air/fuel ratio controller ( 20 ) for controlling the exhaust gas purification system so that an exhaust air/fuel ratio is made rich in a premix combustion mode when it is the time for purifying trapped NOx of the NOx trap catalyst and the NOx trap catalyst is activated (S 13 ) and that the exhaust air/fuel ratio is made rich in a diffusion combustion mode when it is the time for purifying trapped NOx of the Nox trap catalyst and the Nox trap catalyst is not activated (S 15 ). An exhaust gas purification method is also provided.

BACKGROUND OF THE INVENTION

The present invention relates to an exhaust gas purification system foran internal combustion engine, particularly of the kind having a NOxtrap catalyst that traps NOx in an exhaust gas when an exhaust air/fuelratio is lean and releases the trapped NOx when the exhaust air/fuelratio is rich, thereby purifying the exhaust gas. The present inventionfurther relates to an exhaust gas purification method for an internalcombustion engine.

In the diesel engine, when injection of fuel into a cylinder is started,injected fuel is vaporized and formed into a mixture. The mixture thatexists within the cylinder when the temperature and pressure within thecylinder reach predetermined values performs an initial combustion(premix combustion). By this combustion, the temperature and pressurewithin the cylinder become high, so that after the initial combustionthe injected fuel is vaporized and burnt (diffusion combustion)simultaneously with the injection. Since the period (ignition delayperiod) from start of fuel injection to occurrence of the premixcombustion is generally short with respect to the entire combustionperiod, the diffusion combustion is a main combustion.

In contrast to this, Japanese Patent 2864896 discloses a technique forholding down the combustion temperature to low by increasing the EGRrate considerably, while retarding the fuel injection timing to a pointafter the top dead center thereby making an ignition delay periodconsiderably longer and allowing all the quantity of fuel to be injectedwithin the ignition delay period for allowing a premix combustion toserve as a main combustion, thereby reducing the NOx and smoke in theexhaust gas.

In the meantime, a combustion condition wherein a main combustion is adiffusion combustion (this is herein referred to simply as diffusioncombustion or diffusion combustion mode) and a combustion mode wherein amain combustion is a premix combustion (this is herein referred tosimply as premix combustion or premix combustion mode) are generallyswitched from one to another depending upon a driving condition (forexample, in FIG. 8, a zone with EGR corresponds to a premix combustionzone and a zone without EGR corresponds to a diffusion combustion zone).

On the other hand, Japanese Patent 3079933 discloses that in case anexhaust passage is provided with a NOx trap catalyst and at the time forpurifying the trapped NOx, the combustion mode of the diesel engine isswitched from the diffusion combustion mode to the premix combustionmode thereby lowering the excess air rate and reducing and purifying thetrapped NOx.

SUMMARY OF THE INVENTION

The technique disclosed by Japanese Patent 3079933 is adapted to executea rich spike control during the premix combustion at the time ofpurification of NOx trapped in the NOx trap catalyst. However, thetechnique has a problem that even when the exhaust air/fuel ratio ismade rich when the vehicle is in a low-load running condition, asufficient exhaust purification effect by the rich spike control cannotbe attained since the exhaust gas temperature remains low and the lowexhaust gas temperature causes the NOx purifying efficiency to bemaintained low, i.e., the catalyst is not sufficiently activated.

It is accordingly an object of the present invention to provide anexhaust gas purification system for an internal combustion engine thatcan attain an improved exhaust gas purifying ability by executingpurification of trapped NOx of the NOx trap catalyst depending upon anactivated condition of the NOx trap catalyst.

It is a further object of the present invention to provide an exhaustgas purification system that is executed by the exhaust gas purificationsystem of the foregoing character.

According to an aspect of the present invention, there is provided anexhaust gas purification system for an internal combustion engineincluding a NOx trap catalyst disposed in an exhaust passage of theengine for trapping NOx in an inflow exhaust gas when an air/fuel ratioof the exhaust gas is lean and releasing trapped NOx when the air/fuelratio of the exhaust gas is rich, and a control unit for controlling theNOx trap catalyst in accordance with an activated condition of the NOxtrap catalyst, the control unit comprising a purification timedetermining section for determining whether it is the time for purifyingNOx trapped by the NOx trap catalyst, a catalyst activation determiningsection for determining whether the Nox trap catalyst is activated, acombustion mode switching section for switching a combustion mode of theengine between a diffusion combustion mode and a premix combustion modein accordance with an operating condition of the engine, and an air/fuelratio control section for controlling an exhaust air/fuel ratio so thatthe exhaust air/fuel ratio is made rich in the premix combustion modewhen it is the time for purifying trapped NOx of the NOx trap catalystand the NOx trap catalyst is activated and that the exhaust air/fuelratio is made rich in the diffusion combustion mode when it is the timefor purifying trapped NOx of the Nox trap catalyst and the Nox trapcatalyst is not activate.

According to another aspect of the present invention, there is providedan exhaust gas purification method for an internal combustion enginehaving a NOx trap catalyst disposed in an exhaust passage for trappingNOx in an inflow exhaust gas when an air/fuel ratio of the exhaust gasis lean and releasing trapped NOx when the air/fuel ratio of the exhaustgas is rich, the exhaust gas purification method comprising determiningwhether it is the time for purifying NOx trapped by the NOx trapcatalyst, determining whether the Nox trap catalyst is activated,switching a combustion mode of the engine between a diffusion combustionmode and a premix combustion mode in accordance with an operatingcondition of the engine, and controlling an exhaust air/fuel ratio sothat the exhaust air/fuel ratio is made rich in the premix combustionmode when it is the time for purifying trapped NOx of the NOx trapcatalyst and the NOx trap catalyst is activated and that the exhaustair/fuel ratio is made rich in the diffusion combustion mode when it isthe time for purifying trapped NOx of the Nox trap catalyst and the Noxtrap catalyst is not activated.

According to a further aspect of the present invention, there isprovided an exhaust gas purification system for an internal combustionengine comprising a NOx trap catalyst disposed in an exhaust passage ofthe engine for trapping NOx in an inflow exhaust gas when an air/fuelratio of the exhaust gas is lean and releasing trapped NOx when theair/fuel ratio of the exhaust gas is rich, purification time determiningmeans for determining whether it is the time for purifying NOx trappedby the NOx trap catalyst, catalyst activation determining means fordetermining whether the Nox trap catalyst is activated, combustion modeswitching means for switching a combustion mode of the engine between adiffusion combustion mode and a premix combustion mode, and air/fuelratio control means for controlling so that the exhaust air/fuel ratiois made rich in the premix combustion mode when it is the time forpurifying trapped NOx of the NOx trap catalyst and the NOx trap catalystis activated and that the exhaust air/fuel ratio is made rich in thediffusion combustion mode when it is the time for purifying trapped NOxof the Nox trap catalyst and the Nox trap catalyst is not activated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a diesel engine having an exhaust gaspurification system according to an embodiment of the present invention;

FIG. 2 is a flowchart of an exhaust gas purification control executed bythe exhaust gas purification system of FIG. 1;

FIG. 3 is a flowchart of an exhaust gas purification control continuedfrom FIG. 2;

FIG. 4 is a flowchart of an exhaust gas purification control continuedfrom FIG. 2;

FIG. 5 is a diagram showing performance characteristics of an enginewhen combustion is in a rich condition in a diffusion combustion and ina rich condition in a premix combustion;

FIG. 6 is a diagram showing a relation between exhaust gas temperatureand NOx purification ratio;

FIG. 7 is a diagram showing a target intake air quantity at richoperation in a premix combustion;

FIG. 8 is a diagram showing a target EGR rate at rich operation in apremix combustion;

FIG. 9 is a diagram showing a target intake air quantity at richoperation in a diffusion combustion; and

FIG. 10 is a diagram showing a target DPF inlet λ for PM combustion.

DSETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, intake air supplied into diesel engine 1 by way ofintake pipe 2 is supercharged by an intake air compressor of variablenozzle type turbocharger 3. The intake air is cooled by intercooler 4and then flows through intake throttle valve 5 and collector 6 into acombustion chamber of each cylinder. Fuel is pressurized by highpressure fuel pump 7 and conveyed to common rail 8. Then, the fuel isinjected from fuel injector 9 at each cylinder into the combustionchamber. A mixture of air and fuel thus supplied into the combustionchamber is burnt by compression ignition and an exhaust gas is emittedthrough exhaust passage 10.

A portion of the exhaust gas having flown into exhaust passage 10 iscaused to flow back to an intake side through EGR pipe 11 and by way ofEGR control valve 12. The remainder of the exhaust gas is caused to passan exhaust turbine of variable nozzle type turbocharger 3 to drive theexhaust turbine.

In this instance, disposed downstream of the exhaust turbine forpurification of the exhaust gas is NOx trap catalyst 13 that traps NOxin the exhaust gas when the exhaust air/fuel ratio is lean anddischarges or reduces NOx when the exhaust air-fuel ratio is rich.Further, NOx trap catalyst 13 carries thereon an oxidation catalyst soas to have a function of oxidizing HC and CO in the exhaust gas, thusbeing adapted to serve as a NOx trap catalyst with an oxidationfunction.

Further, disposed downstream of NOx trap catalyst 13 is dieselparticulate filter (DPF) 14 for scavenging particular matter (PM) in theexhaust gas. Further, DPF 14 carries thereon three way catalyst so as tohave a function of oxidizing HC and CO in the exhaust gas and a functionof reducing NOx in the exhaust gas, thus being adapted to constitute aDPF with a three way catalytic function.

Inputted to control unit 20 are signals from engine speed sensor 21 fordetecting engine speed Ne, accelerator opening degree sensor 22 fordetecting accelerator opening degree APO, etc. for controlling engine 1.

Further, in this embodiment, there are provided catalyst temperaturesensor 23 for detecting the temperature (catalyst temperature) of NOxtrap catalyst 13, exhaust pressure sensor 24 for detecting the exhaustpressure at an inlet side of DPF 14 in exhaust passage 10, DPFtemperature sensor 25 for detecting the temperature (DPF temperature) ofDPF 14 and oxygen concentration sensor 26 for detecting an oxygenconcentration at an outlet side of DPF 14 in exhaust passage 10. Signalsfrom those sensors are also inputted to control unit 20. However, thetemperature of NOx trap catalyst 13 and the temperature of DPF 14 can bedetected by detecting the temperature of the exhaust gas presentadjacent NOx trap catalyst 13 and DPF 14.

Based on those signals, control unit 20 outputs to fuel injector 9 afuel injection command signal for controlling a fuel injection quantityfor a main injection and a pilot injection prior to the main injectionand fuel injection timing of fuel injector 9, to intake throttle valve aopening degree command signal, to EGR control valve 12 a opening degreecommand signal, etc.

The exhaust gas purification system of this embodiment is adapted toexecute an exhaust gas purification control for releasing NOx trapped byand accumulated on NOx trap catalyst, burning thereby removing sulfur(S) accumulated on NOx trap catalyst13 due to S-poisoning (removal ofS-poisoning), and burning thereby removing PM scavenged by DPF 14(recovery of DPF). The exhaust gas purification control will bedescribed in detail hereinafter.

FIGS. 2 to 4 show a flowchart of an exhaust gas purification controlthat is executed in control unit 20.

In step S1, an engine operating condition (engine speed Ne andaccelerator opening degree APO) is read from engine speed sensor 21 andaccelerator opening degree sensor 22.

In step S2, the amount of NOx trapped by and accumulated on NOx trapcatalyst 13 is detected. However, since it is difficult to detect theaccumulated amount of NOx directly, the accumulated amount of NOx isdetected indirectly by estimating the generated amount of NOx per unittime on the basis of engine speed Ne and fuel injection quantity Q,obtaining the accumulated amount of NOx per unit time on considerationof an NOx trapping rate and summing up the accumulated amount of NOx perunit time. Of course, in case the accumulated amount of NOx is obtainedby summing up in the above-described manner, the summed up value isreset or cleared when the NOx purification process that will bedescribed hereinafter is executed.

In step S4, the accumulated amount of sulfur on NOx trap catalyst 13 dueto S-poisoning is detected. However, it is difficult to detect theaccumulated amount of sulfur directly, the accumulated amount of sulfuris detected indirectly in the following manner. The accumulated amountof sulfur is determined depending upon the concentration of sulfur inthe exhaust gas. Thus, it is considered that the accumulated amount ofsulfur can be determined depending upon the fuel consumption, providedthat fuel is the same. Accordingly, the fuel consumption is detected andthe accumulated amount of sulfur can be estimated based on the fuelconsumption. Further, since the travel distance increases with increaseof the consumed amount of fuel, the consumed amount of fuel and thetravel distance can be considered equivalent to each other. Accordingly,the accumulated amount of sulfur is herein estimated depending upon thetravel distance.

In step S5, the amount of PM scavenged by and accumulated on DPF 14 isdetected. However, since it is difficult to detect the accumulatedamount of PM directly, the accumulated amount of PM is detectedindirectly in the following manner. Increase of the accumulated amountof PM on DPF 14 naturally causes the upstream side exhaust pressure ofDPF 14 to become higher. Accordingly, the upstream side exhaust pressureof DPF 14 is detected by exhaust pressure sensor 24 and compared withthe reference exhaust pressure at the present operating condition(engine speed Ne and fuel injection quantity Q) thereby estimating theaccumulated amount of PM.

In step S6, it is determined whether premix rich spike condition flagSP1 for NOx purification of NOx trap catalyst 13 is set to 1. If SP1=1,the program proceeds to step S7.

In step S7, it is determined whether diffusion rich spike condition flagSP2 for NOx purification of NOx trap catalyst 13 is set to 1. If SP2=0,the program proceeds to step S8.

In step S8, it is determined whether recovery temperature rise flag reg1for recovery of DPF 14 (or release of NOx trap catalyst 13 fromS-poisoning) is set to 1. If reg1=0, the program proceeds to step S9.

In step S9, it is determined whether recovery combustion flag reg2 forrecovery of DPF 14 (or release from S-poisoning of NOx trap catalyst) isset to 1. If reg2=0, the program proceeds to step 10.

In step S10, it is determined whether the accumulated amount of NOx ofNOx trap catalyst 13 is equal to or larger than a predetermined valueNOx1. If the accumulated amount of NOx of NOx trap catalyst 13 issmaller than the predetermined value NOx1, the program proceeds to stepS17.

In step S17, it is determined whether the accumulated amount of sulfurof NOx trap catalyst 13 that is detected in step S4 becomes equal to orlarger than a predetermined value S1 (whether the vehicle has traveled apredetermined distance). If the accumulated amount of sulfur of NOx trapcatalyst is smaller than the predetermined value S1, the programproceeds to step S18.

In step S18, it is determined whether the accumulated amount of PM ofDPF 14 that is detected at step S5 becomes equal to a predeterminedvalue PM1 (whether the upstream side exhaust pressure of DPF 14 exceedsan exhaust pressure threshold value that corresponds to the presentengine operating condition). If the accumulated amount of PM of DPF 14is smaller than the predetermined value PM1, this flow is ended.

In the meantime, if any particular control is not executed by this flow,switching between a diffusion combustion mode and a premix combustionmode is performed in accordance with an operating condition of engine 1.Specifically, as shown in FIG. 8, a premix combustion is performed whenengine 1 is in the low to middle speed and low to middle load rangewhere EGR is executed and a diffusion combustion is performed in thehigh-speed high-load range where EGR is not executed.

Then, description is made as to the case where it is determined in stepS10 that the accumulated amount of NOx of NOx trap catalyst 13 hasbecome equal to or larger than the predetermined value NOx1 (i.e., thetime for purification of NOx of NOx trap catalyst 13 has come), the casewhere it is determined in step S17 that the accumulated amount of sulfurof NOx trap catalyst 13 has become equal to or larger than thepredetermined value S1 (i.e., the time for release from S-poisoning hascome), and the case where it is determined in step S18 that theaccumulated amount of PM of DPF 14 has become equal to or larger thanthe predetermined value PM1 (i.e., the time for recovery of DPF hascome).

[In Case it is the Time for NOx Purification of NOx Trap Catalyst]

If it is determined in step S10 that the accumulated value of NOx of Noxtrap catalyst 13 is equal to or larger than the predetermined valueNOx1, the program proceeds to step S11 for purifying NOx of NOx trapcatalyst 13.

Herein, description is first made as to purification of NOx of NOx trapcatalyst.

For NOx purification of NOx trap catalyst 13, a rich operation of engine1 is executed to release NOx trapped by and accumulated on NOx trapcatalyst 13.

If a rich operation of engine 1 is executed during the premix combustion(large amount of EGR and high swirl), the exhaust temperature is loweras compared with that at a rich operation during the diffusioncombustion where the ignition delay period is short, as shown in FIG. 5.Conversely speaking, if a rich condition in the diffusion combustion isrealized, the exhaust temperature can be made higher as compared withthat in the rich condition in the premix combustion.

However, the rich condition in the diffusion combustion increases thefuel consumption ratio, thus deteriorating the fuel consumption.Further, NOx trap catalyst 13 may crack if heated excessively, thusbeing deteriorated in the trapping and purifying ability.

Accordingly, for purification of NOx trapped by NOx trap catalyst 13when the activity of catalyst 13 is sufficiently high, a rich spikeoperation in the premix combustion is preferred since it can suppressdeterioration of fuel consumption and catalyst 13.

In contrast to this, if there is a rich spike command when the activityof catalyst 13 is low, a rich spike operation in the diffusioncombustion that can attain a higher exhaust temperature is executed.This makes it possible to improve the NOx purification ratio. However,there is apprehension that a rich spike in the diffusion combustioncauses deteriorated combustion noise. The combustion noise can besuppressed by performing pilot injection.

On the condition described above, a control in step S11 onward will beexecuted.

In step S11, the catalyst temperature detected by catalyst temperaturesensor 23 is read and it is determined whether the catalyst temperatureis equal to or higher than a predetermined temperature Tlow that is setat a value lower than an activation temperature of catalyst 13.

In step S12, it is further determined, when the catalyst temperature isequal to or higher than the predetermined temperature Tlow, whether thecatalyst temperature is equal to or higher than a predeterminedtemperature Tdif that corresponds to the activation temperature of NOxtrap catalyst 13.

By this, cases are divided into three, i.e., a case where the catalysttemperature≧Tdif, a case where Tlow≦the catalyst temperature<Tdif and acase where the catalyst temperature<Tlow.

In this instance, as shown in FIG. 6, the purification ratio of NOx ofNOx trap catalyst 13 is sufficiently large when the catalyst temperatureis in the range higher than Tdif. However, the purification ratio of NOxbecomes lower when the catalyst temperature becomes lower than Tdif andbecomes further lower to such an extend that purification of NOx cannotbe expected when the catalyst temperature becomes lower than Tlower.

Accordingly, if the catalyst temperature≧Tdif (i.e., when thedetermination in step S12 is affirmative), it is determined that NOxtrap catalyst 13 is in a sufficiently activated state and the programproceeds to step S13. In step S13, a rich operation is executed in thepremix combustion thereby purifying NOx while preventing a deteriorationof fuel consumption and catalyst.

In this instance, in step S13, a target intake air quantity and targetEGR ratio for realizing a rich operation in the premix combustion areset by using engine speed Ne and fuel injection quantity Q as parametersof an engine operating condition and by making reference to the targetintake air quantity map of FIG. 7 and the target EGR map of FIG. 8.Based on the thus set target intake air quantity and target EGR ratio,intake throttle valve 15 and EGR control valve 12 are controlled therebyrealizing a rich operation in the premix combustion. In step S14, premixrich spike condition flag SP1 is set to 1 and this flow is ended.

Further, if Tlow≦catalyst temperature<Tdif (i.e., when the determinationin step S12 is negative), it is determined that NOx trap catalyst 13 isnot in a sufficiently activated state, and the program proceeds to stepS15. In step S15, a rich operation is executed in the diffusioncombustion that is higher in exhaust temperature than the premixcombustion. This causes the exhaust temperature to rise up to a valueequal to or higher than Tdif, thus making it possible execute to executea rich spike operation thereby improving the NOx purification ratio.

In this instance, in step S15, a target intake air quantity is set byreference to the target intake air quantity map of FIG. 9 for realizinga rich operation in the diffusion combustion. Based on the thus settarget intake air quantity, intake throttle valve 5 is controlled whileat the same time a pilot injection is executed by fuel injector 9,thereby realizing a rich operation in the diffusion combustion. In stepS16, diffusion rich spike condition flag SP2 is set to 1, and this flowis ended.

Further, if catalyst temperature<Tlow (i.e., when the determination instep S11 is negative), this flow is ended. In this instance, since theactivated condition of NOx trap catalyst 13 is bad so that it isdifficult to elevate the catalyst temperature beyond Tdif even if a richoperation in the diffusion combustion is executed, i.e., a rich spikeoperation under the condition of Tlow≦catalyst temperature<Tdif haslittle effect on the purification of NOx, a rich spike operation is notexecuted and emission of NOx is suppressed only by the premixcombustion. The exhaust temperature at the time of the premix combustionranges from Tlow to Tdif with exceptions so that during the time thepremix combustion is continued the catalyst temperature becomes higherto exceed Tlow.

In case a rich operation in the premix combustion is started (S13),premix rich spike condition flag SP1 is set to 1 in step S14. Thus, fromthe next time, after the determination in step S6, the program proceedsto step S21 onward (FIG. 3) where a rich operation in the premixcombustion is continued for a predetermined time.

In step S21, it is determined whether a predetermined time tsp1necessary for NOx purification has elapsed or not. If predetermined timetsp1 has not yet elapsed, this flow is ended at once. If predeterminedtime tsp1 has elapsed, the rich operation in the premix combustion iscancelled in step S22 and premix rich spike condition flag sp1 is set to0 in step S23. Thereafter, this flow is ended.

Further, in case a rich operation in the diffusion combustion is started(S15), diffusion rich condition flag SP2 is set to 1 in step S16. Thus,from the next time, after the determination in step S7, the programproceeds to step S24 onward (FIG. 3) where a rich operation in thediffusion combustion is continued for a predetermined time.

In step S24, it is determined whether predetermined time tsp2 necessaryfor NOx purification has elapsed. If predetermined time tsp2 has not yetelapsed, this flow is ended at once. If predetermined time tsp2 haselapsed, the rich operation in the diffusion combustion is cancelled instep S25 and diffusion rich spike condition flag sp2 is set to 0 in stepS26. Thereafter, this flow is ended.

[When it is the Time for Recovery from S-poisoning] and [When it is theTime for Recovery of DPF]

If it is determined in step S17 that the accumulated amount of sulfur ofNOx trap catalyst 13 is equal to or larger than a predetermined valueS1, the program proceeds to step S19 and step S20 for recovery fromS-poisoning.

If it is determined in step S18 that the accumulated amount of PM of DPF14 is equal to or larger than a predetermined value PM1, the programproceeds to step S19 and step S20 for recovery of DPF 14.

Herein, description is first made as to recovery of DPF 14 and recoveryof NOx trap catalyst from S-poisoning.

For recovery of DPF and recovery from S-poisoning, a rich operation isexecuted so as to elevate the exhaust temperature thereby elevating thetemperature of DPF 14 above a combustible temperature of PM. In themeantime, a combustible temperature of sulfur is lower than thecombustible temperature of PM. Further, since in this embodiment NOxtrap catalyst 13 on which sulfur is accumulated is located upstream ofDPF 14, recovery of NOx trap catalyst 13 from S-poisoning can beattained by elevating the temperature of DPF 14 above the combustibletemperature of PM.

If a rich operation is executed in the premix combustion (large amountof EGR+high swirl), the resulting exhaust temperature is lower ascompared with that in a rich condition in diffusion combustion whoseignition delay period is short, due to specific heat, etc. of workinggas provided by EGR. In other words, if a rich condition is realizedduring the diffusion combustion, a higher exhaust temperature can beattained as compared with that in the rich condition in the premixcombustion.

Since a high exhaust temperature (i.e., >600° C.) is required forrecovery of DPF 14 and recovery of NOx trap catalyst from S-poisoning,it is desirable to elevate the temperature of DPF 14 by a rich conditionin the diffusion combustion by which a higher exhaust temperature can beexpected. However, if a rich spike control is executed during thediffusion combustion, there is an apprehension that a combustion noiseis deteriorated. The combustion noise can be suppressed by executingpilot injection.

Accordingly, in step S19, since it has already been determined that itis the time for recovery of DPF 14 or for recovery of NOx trap catalyst13 from S-poisoning, a rich operation in the diffusion combustion bywhich a higher exhaust temperature can be expected is executed with aview to elevating the temperature of DPF 14 above the combustibletemperature of PM. Also in this instance, pilot injection is executed.

Then, in step S20, recovery temperature rise flag reg1 is set to 1, andthis flow is ended.

By this, from the next time, since it is determined in step S8 thatreg1=1, the program proceeds to step S27 onward.

In step S27, when the temperature of DPF 14 is elevated by a richcondition in the diffusion combustion, the oxygen concentration at theoutlet of DPF 14, i.e., the exhaust air/fuel ratio (DPF outlet λ) isdetected by oxygen concentration sensor 26 and intake throttle valve 5is controlled so that the exhaust air/fuel ratio becomes stoichiometric.In this instance, if DPF 14 carries thereon a three-way catalyst, athree-way catalytic function can be realized by controlling so that theDPF outlet λ becomes stoichiometric, thus making it possible to continuepurification of exhaust gas even during elevation of the temperature ofDPF 14.

In step S28, the temperature of DPF 14 detected by DPF temperaturesensor 25 is read and it is determined whether the temperature of DPF 14exceeds a combustible temperature T2 of PM.

If the temperature of DPF 14 has not yet exceeded T2, this flow is endedat once and the rich operation in the diffusion combustion under the DPFoutlet λ control is continued. If the temperature of DPF 14 exceeds T2,the program proceeds to step S29.

In step S29, since the temperature of DPF 14 has reached temperature T2for allowing PM to be combustible, the DPF outlet λ control is ended.

In step S30, the exhaust air/fuel ratio at the inlet of DPF 14 (DPFinlet λ) is started. Namely, as shown in FIG. 10, by reference to a mapthat uses engine speed Ne and fuel injection quantity Q as parameters ofan engine operating condition and defines target DPF inlet λ for burningthereby removing PM, intake throttle valve 15 is controlled so as toobtain the target DPF inlet λ thereby supplying a predetermined amountof oxygen to DPF 14 and burning out the PM accumulated on DPF 14. Sincethe temperature of DPF 14 has already reached the combustibletemperature of PM, supply of oxygen to DPF 14 causes the PM accumulatedon DPF 14 to burn all at once. In the meantime, since by controlling theDPF inlet λ the combustion speed of PM can be controlled, thus making itpossible to prevent melting and burning of DPF 14.

In step S31, since DPF 14 has been out of the recovery temperature risecondition, the recovery temperature rise flag reg1 is set to 0.

In step S32, since DPF 14 has been in a recovery combustion condition,recovery combustion flag reg2 is set to 1 and this flow is ended.

By this, from the next time, since it has been determined in step S9that reg2=1, the program proceeds to step S33 (FIG. 4).

In step S33, the temperature of DPF 14 detected by DPF temperaturesensor 25 is read and it is determined whether the temperature of DPF 14is lower than temperature T1 that does not cause abnormal burning sincecombustion of PM at DPF 14 has been almost finished.

If the temperature of DPF 14 has not yet fallen to T1, this flow isended at once and the DPF inlet λ control is continued. This is becausedue to the possibility of abnormal combustion, it is necessitated tocontinue the DPF inlet λ control thereby holding the DPF inlet λ at apredetermined value until the temperature of DPF 14 is lowered beyondtemperature T1.

If the temperature of DPF 14 has fallen beyond T1, the program proceedsto step S34.

In step S34, the DPF inlet λ control is ended.

In step S35, since DPF 14 has been out of the recovery combustioncondition, the recovery combustion flag reg2 is set to 0 and this flowis ended.

In the foregoing, it is to be noted that step S11 of the program storedin control unit 20 corresponds to a purification time determiningsection for determining whether it is the time for purifying NOx trappedby NOx trap catalyst 13.

It is further to be noted that step S12 of the program stored in controlunit 20 corresponds to an activated condition determining section fordetermining an activated condition of NOx trap catalyst 13.

It is further to be noted that steps S13 and S15 of the program storedin control unit 20 constitute an air/fuel ratio control section forcontrolling an exhaust air/fuel ratio for purifying trapped NOx of NOxtrap catalyst 13 and a combustion mode switching section for switching acombustion mode of engine 1.

It is further to be noted that step S18 of the program stored in controlunit 20 constitutes a recovery time determining section for determiningwhether it is the time for executing recovery of DPF 14.

It is further to be noted that step S17 of the program stored in controlunit 20 corresponds to a S-poisoning recovery time determining fordetermining whether it is the time for executing recovery of Nox trapcatalyst from S-poisoning.

It is further to be noted that step 19 of the program stored in controlunit 20 corresponds to an air/fuel ratio control section for controllingan exhaust air/fuel ratio for recovery of NOx trap catalyst 13 fromS-poisoning.

From the foregoing, it will be understood that when trapped NOx of NOxtrap catalyst 13 is to be purified and the activity of catalyst 13 islow due to a low exhaust temperature, a rich operation is executed in adiffusion combustion mode thereby causing the exhaust temperature torise sharply and catalyst 13 to be activated, thus making it possible topurify NOx accumulated on catalyst 13, i.e., release trapped NOx fromcatalyst 13. Further, under the condition where the catalyst temperatureis high and catalyst 13 is in a sufficiently activated condition, a richoperation is executed in a premix combustion mode. This makes itpossible to prevent a deterioration of fuel consumption and protectcatalyst 13 from excessive heating thereby suppressing a deteriorationof catalyst 13.

It will be further understood that in a premix combustion mode that isattained by lowering the exhaust temperature and making longer theignition delay period, a rich condition can be set by enhancing thedegree of the premix combustion, thus making it possible to set a richcondition without incurring a deterioration of smoke. In addition, sincethere is not caused any variation of combustion between the time whenthe premix combustion is rich and the time when the premix combustion islean, it becomes possible to set a rich condition of the premixcombustion while suppressing a variation of combustion noise resultingat the time of switching from a rich condition to a lean condition orvice versa.

It will be further understood that for recovery of filter 14 forscavenging PM in an exhaust gas, i.e., for burning and thereby removingPM of the exhaust gas scavenged and accumulated by DPF 14, a highexhaust temperature under a condition of low oxygen concentration isrequired. Thus, temperature of DPF 14 is elevated under a rich conditionin the diffusion combustion mode. This makes it possible to elevate thetemperature of DPF 14 to a target value within a short time and reduce adeterioration of fuel consumption to a minimum.

The entire contents of Japanese Patent Application 2001-254517 areincorporated herein by reference.

Although the invention has been described above by reference to acertain embodiment of the invention, the invention is not limited to theembodiment described above. Modifications and variations of theembodiment described above will occur to those skilled in the art, inlight of the above teachings. The scope of the invention is defined withreference to the following claims.

What is claimed is:
 1. An exhaust gas purification system for aninternal combustion engine including: a NOx trap catalyst disposed in anexhaust passage of the engine for trapping NOx in an inflow exhaust gaswhen an air/fuel ratio of the exhaust gas is lean and releasing trappedNOx when the air/fuel ratio of the exhaust gas is rich; and a controlunit for controlling the NOx trap catalyst in accordance with anactivated condition of the NOx trap catalyst; the control unitcomprising: a purification time determining section for determiningwhether it is the time for purifying NOx trapped by the NOx trapcatalyst; a catalyst activation determining section for determiningwhether the Nox trap catalyst is activated; a combustion mode switchingsection for switching a combustion mode of the engine between adiffusion combustion mode and a premix combustion mode in accordancewith an operating condition of the engine; and an air/fuel ratio controlsection for controlling an exhaust air/fuel ratio so that the exhaustair/fuel ratio is made rich in the premix combustion mode when it is thetime for purifying trapped NOx of the NOx trap catalyst and the NOx trapcatalyst is activated and that the exhaust air/fuel ratio is made richin the diffusion combustion mode when it is the time for purifyingtrapped NOx of the Nox trap catalyst and the Nox trap catalyst is notactivate.
 2. The exhaust gas purification system according to claim 1,wherein the combustion mode switching section comprises a combustiontemperature control section for lowering a combustion temperature of theengine in accordance with an operating condition of the engine, and anignition delay period control section for making considerably longer anignition delay period during the time the combustion temperature controlsection is in operation so that a heat generation pattern by one of thepremix combustion mode and the diffusion combustion mode is attained,the premix combustion mode being attained by the combustion temperaturecontrol section and the ignition delay period control section.
 3. Theexhaust gas purification system according to claim 1, further comprisinga filter disposed in the exhaust passage for scavenging particulatematter (PM) in the exhaust gas, the control unit further including arecovery time determining section for determining whether it is the timefor executing recovery of the filter, wherein the control unit furthercomprises an air/fuel ratio control section for controlling an exhaustair/fuel ratio so that the exhaust air/fuel ratio is made rich in thediffusion combustion mode when it is the time for executing recovery ofthe filter.
 4. The exhaust gas purification system according to claim 3,wherein the filter has at least one of a catalytic function ofexhibiting an oxidizing ability when placed in a lean atmosphere and areducing ability when placed in a rich atmosphere and a catalyticfunction of absorbing NOx when placed in a lean atmosphere and reducingthereby purifying absorbed NOx when placed in a rich atmosphere.
 5. Theexhaust gas purification system according to claim 1, wherein thecatalyst activation determining section has a detecting section fordetecting a temperature of the NOx trap catalyst and determines whetherthe NOx trap catalyst is activated, based on the temperature of the NOxtrap catalyst.
 6. The exhaust gas purification system according to claim1, wherein the catalyst activation determining section has an estimatingsection for estimating a temperature of the NOx trap catalyst anddetermines whether the NOx trap catalyst is activated, based on anestimated temperature of the NOx trap catalyst.
 7. The exhaust gaspurification system according to claim 1, wherein the air/fuel ratiocontrol section executes a pilot injection when the exhaust air/fuelratio is made rich in the diffusion combustion mode.
 8. The exhaust gaspurification system according to claim 2, further comprising an intakethrottle valve and an EGR control valve of the engine, the air/fuelratio control section controlling the exhaust air/fuel ratio by means ofthe intake throttle valve and the EGR control valve when the exhaustair/fuel ratio is made rich in the premix combustion mode.
 9. Theexhaust gas purification system according to claim 1, wherein thecontrol unit further comprises a S-poisoning recovery time determiningsection for determining whether it is the time for executing recovery ofthe NOx trap catalyst from S-poisoning, and an air/fuel ratio controlsection for controlling the exhaust air/fuel ratio so that the exhaustair/fuel ratio is made rich in the diffusion combustion mode when it isthe time for executing recovery of the NOx trap catalyst fromS-poisoning.
 10. An exhaust gas purification system for an internalcombustion engine comprising: a NOx trap catalyst disposed in an exhaustpassage of the engine for trapping NOx in an inflow exhaust gas when anair/fuel ratio of the exhaust gas is lean and releasing trapped NOx whenthe air/fuel ratio of the exhaust gas is rich; purification timedetermining means for determining whether it is the time for purifyingNOx trapped by the NOx trap catalyst; catalyst activation determiningmeans for determining whether the Nox trap catalyst is activated;combustion mode switching means for switching a combustion mode of theengine between a diffusion combustion mode and a premix combustion mode;and air/fuel ratio control means for controlling so that the exhaustair/fuel ratio is made rich in the premix combustion mode when it is thetime for purifying trapped NOx of the NOx trap catalyst and the NOx trapcatalyst is activated and that the exhaust air/fuel ratio is made richin the diffusion combustion mode when it is the time for purifyingtrapped NOx of the Nox trap catalyst and the Nox trap catalyst is notactivated.
 11. An exhaust gas purification method for an internalcombustion engine having a NOx trap catalyst disposed in an exhaustpassage for trapping NOx in an inflow exhaust gas when an air/fuel ratioof the exhaust gas is lean and releasing trapped NOx when the air/fuelratio of the exhaust gas is rich, the exhaust gas purification methodcomprising: determining whether it is the time for purifying NOx trappedby the NOx trap catalyst; determining whether the Nox trap catalyst isactivated; switching a combustion mode of the engine between a diffusioncombustion mode and a premix combustion mode in accordance with anoperating condition of the engine; and controlling an exhaust air/fuelratio so that the exhaust air/fuel ratio is made rich in the premixcombustion mode when it is the time for purifying trapped NOx of the NOxtrap catalyst and the NOx trap catalyst is activated and that theexhaust air/fuel ratio is made rich in the diffusion combustion modewhen it is the time for purifying trapped NOx of the Nox trap catalystand the Nox trap catalyst is not activated.
 12. The exhaust gaspurification method according to claim 11, wherein the switching of thecombustion mode comprises lowering a combustion temperature of theengine in accordance with an operating condition of the engine, andmaking considerably longer an ignition delay period during the time thecombustion temperature control section is in operation so that a heatgeneration pattern by one of the premix combustion mode and thediffusion combustion mode is attained, the premix combustion beingattained by the lowering of the combustion temperature and the makingconsiderably longer of the ignition delay period.
 13. The exhaust gaspurification method according to claim 11, wherein the engine furthercomprises a filter disposed in the exhaust passage for scavengingparticulate matter (PM) in the exhaust gas, and wherein the exhaust gaspurification method further comprises determining whether it is the timefor executing recovery of the filter, and controlling an exhaustair/fuel ratio so that the exhaust air/fuel ratio is made rich in thediffusion combustion mode when it is the time for executing recovery ofthe filter.
 14. The exhaust gas purification method according to claim11, wherein the determining whether the NOx trap catalyst is activatedcomprises detecting a temperature of the NOx trap catalyst anddetermining whether the NOx trap catalyst is activated, based on thetemperature of the NOx trap catalyst.
 15. The exhaust gas purificationmethod according to claim 11, wherein the determining whether the NOxtrap catalyst is activated comprises estimating a temperature of the NOxtrap catalyst and determining whether the NOx trap catalyst isactivated, based on an estimated temperature of the NOx trap catalyst.16. The exhaust gas purifying method according to claim 11, wherein thecontrolling of the exhaust air/fuel ratio comprises executing a pilotinjection when the exhaust air/fuel ratio is made rich in the diffusioncombustion mode.
 17. The exhaust gas purifying method according to claim12, wherein the engine further comprises an intake throttle valve and anEGR control valve, and wherein the controlling of the exhaust air/fuelratio comprises controlling the exhaust air/fuel ratio by means of theintake throttle valve and the EGR control valve when the exhaustair/fuel ratio is made rich in the premix combustion mode.
 18. Theexhaust gas purification method according to claim 11, further comprisesdetermining whether it is the time for executing recovery of the NOxtrap catalyst from S-poisoning, and controlling the exhaust air/fuelratio so that the exhaust air/fuel ratio is made rich in the diffusioncombustion mode when it is the time for executing recovery of the NOxtrap catalyst from S-poisoning.
 19. An exhaust gas purification systemfor an internal combustion engine including: a NOx trap catalystdisposed in an exhaust passage of the engine for trapping NOx in aninflow exhaust gas when an air/fuel ratio of the exhaust gas is lean andreleasing trapped NOx when the air/fuel ratio of the exhaust gas isrich; and a control unit for controlling the NOx trap catalyst inaccordance with an activated condition of the NOx trap catalyst; thecontrol unit comprising: a purification time determining section fordetermining whether it is the time for purifying NOx trapped by the NOxtrap catalyst; a catalyst activation determining section for determiningwhether the Nox trap catalyst is activated; a combustion mode switchingsection for switching a combustion mode of the engine between adiffusion combustion mode and a premix combustion mode in accordancewith an operating condition of the engine; and an air/fuel ratio controlsection for controlling an exhaust air/fuel ratio so that the exhaustair/fuel ratio is made rich in the premix combustion mode when it is thetime for purifying trapped NOx of the NOx trap catalyst and the NOx trapcatalyst is activated and that the exhaust air/fuel ratio is made richin the diffusion combustion mode when it is the time for purifyingtrapped NOx of the Nox trap catalyst and the Nox trap catalyst is not inactivated.
 20. The exhaust gas purification system according to claim19, wherein the combustion mode switching section comprises a combustiontemperature control section for lowering a combustion temperature of theengine in accordance with an operating condition of the engine, and anignition delay period control section for making considerably longer anignition delay period during the time the combustion temperature controlsection is in operation so that a heat generation pattern by one of thepremix combustion mode and the diffusion combustion mode is attained,the premix combustion mode being attained by the combustion temperaturecontrol section and the ignition delay period control section.
 21. Theexhaust gas purification system according to claim 19, furthercomprising a filter disposed in the exhaust passage for scavengingparticulate matter (PM) in the exhaust gas, the control unit furtherincluding a recovery time determining section for determining whether itis the time for executing recovery of the filter, wherein the controlunit further comprises an air/fuel ratio control section for controllingan exhaust air/fuel ratio so that the exhaust air/fuel ratio is maderich in the diffusion combustion mode when it is the time for executingrecovery of the filter.
 22. The exhaust gas purification systemaccording to claim 21, wherein the filter has at least one of acatalytic function of exhibiting an oxidizing ability when placed in alean atmosphere and a reducing ability when placed in a rich atmosphereand a catalytic function of absorbing NOx when placed in a leanatmosphere and reducing thereby purifying absorbed NOx when placed in arich atmosphere.
 23. The exhaust gas purification system according toclaim 19, wherein the activation condition determining section has adetecting or estimating section for detecting or estimating atemperature of the NOx trap catalyst and determines whether the NOx trapcatalyst is activated, based on the temperature of the NOx trapcatalyst.
 24. The exhaust gas purification system according to claim 19,wherein the air/fuel ratio control section executes a pilot injectionwhen the exhaust air/fuel ratio is made rich in the diffusion combustionmode.
 25. The exhaust gas purification system according to claim 20,further comprising an intake throttle valve and an EGR control valve ofthe engine, the air/fuel ratio control section controlling the exhaustair/fuel ratio by means of the intake throttle valve and the EGR controlvalve when the exhaust air/fuel ratio is made rich in the premixcombustion mode.
 26. The exhaust gas purification system according toclaim 19, wherein the control unit further comprises a S-poisoningrecovery time determining section for determining whether it is the timefor executing recovery of the NOx trap catalyst from S-poisoning, and anair/fuel ratio control section for controlling the exhaust air/fuelratio so that the exhaust air/fuel ratio is made rich in the diffusioncombustion mode when it is the time for executing recovery of the NOxtrap catalyst from S-poisoning.
 27. An exhaust gas purification systemfor an internal combustion engine comprising: a NOx trap catalystdisposed in an exhaust passage of the engine for trapping NOx in aninflow exhaust gas when an air/fuel ratio of the exhaust gas is lean andreleasing trapped NOx when the air/fuel ratio of the exhaust gas isrich; purification time determining means for determining whether it isthe time for purifying NOx trapped by the NOx trap catalyst; catalystactivation determining means for determining whether the Nox trapcatalyst is activated; combustion mode switching means for switching acombustion mode of the engine between a diffusion combustion mode and apremix combustion mode; and air/fuel ratio control means for controllingso that the exhaust air/fuel ratio is made rich in the premix combustionmode when it is the time for purifying trapped NOx of the NOx trapcatalyst and the NOx trap catalyst is activated and that the exhaustair/fuel ratio is made rich in the diffusion combustion mode when it isthe time for purifying trapped NOx of the Nox trap catalyst and the Noxtrap catalyst is not activated.
 28. An exhaust gas purification methodfor an internal combustion engine having a NOx trap catalyst disposed inan exhaust passage for trapping NOx in an inflow exhaust gas when anair/fuel ratio of the exhaust gas is lean and releasing trapped NOx whenthe air/fuel ratio of the exhaust gas is rich, the exhaust gaspurification method comprising: determining whether it is the time forpurifying NOx trapped by the NOx trap catalyst; determining an activatedcondition of the Nox trap catalyst; switching a combustion mode of theengine between a diffusion combustion mode and a premix combustion modein accordance with an operating condition of the engine; and controllingan exhaust air/fuel ratio so that the exhaust air/fuel ratio is maderich in the premix combustion mode when it is the time for purifyingtrapped NOx of the NOx trap catalyst and the NOx trap catalyst isactivated and that the exhaust air/fuel ratio is made rich in thediffusion combustion mode when it is the time for purifying trapped NOxof the Nox trap catalyst and the Nox trap catalyst is not activated. 29.The exhaust gas purification method according to claim 28, wherein theswitching of the combustion mode comprises lowering a combustiontemperature of the engine in accordance with an operating condition ofthe engine, and making considerably longer an ignition delay periodduring the time the combustion temperature control section is inoperation so that a heat generation pattern by one of the premixcombustion mode and the diffusion combustion mode is attained, thepremix combustion being attained by the lowering of the combustiontemperature and the making considerably longer of the ignition delayperiod.
 30. The exhaust gas purification method according to claim 28,wherein the engine further comprises a filter disposed in the exhaustpassage for scavenging particulate matter (PM) in the exhaust gas, andwherein the exhaust gas purification method further comprisesdetermining whether it is the time for executing recovery of the filter,and controlling an exhaust air/fuel ratio so that the exhaust air/fuelratio is made rich in the diffusion combustion mode when it is the timefor executing recovery of the filter.
 31. The exhaust gas purificationmethod according to claim 28, wherein the determining whether the NOxtrap catalyst is activated comprises detecting or estimating atemperature of the NOx trap catalyst and determining whether the NOxtrap catalyst is activated, based on the temperature of the NOx trapcatalyst.
 32. The exhaust gas purifying method according to claim 28,wherein the controlling of the exhaust air/fuel ratio comprisesexecuting a pilot injection when the exhaust air/fuel ratio is made richin the diffusion combustion mode.
 33. The exhaust gas purifying methodaccording to claim 29, wherein the engine further comprises an intakethrottle valve and an EGR control valve, and wherein the controlling ofthe exhaust air/fuel ratio comprises controlling the exhaust air/fuelratio by means of the intake throttle valve and the EGR control valvewhen the exhaust air/fuel ratio is made rich in the premix combustionmode.
 34. The exhaust gas purification method according to claim 28,further comprises determining whether it is the time for executingrecovery of the NOx trap catalyst from S-poisoning, and controlling theexhaust air/fuel ratio so that the exhaust air/fuel ratio is made richin the diffusion combustion mode when it is the time for executingrecovery of the NOx trap catalyst from S-poisoning.